my @product = matrix_multiply(@matrix1, @matrix2);

##</code><code>##

my @product = @{ matrix_multiply(\@matrix1, \@matrix2) };

##</code><code>##

my @product = matrix_multiply(
   $matrix1[0],
   $matrix1[1],
   $matrix1[2],
   $matrix2[0],
   $matrix2[1],
   $matrix2[2]
);
# @product has only one element.
# $product[0] is a ref to the resulting matrix